Phase locked loops (PLLs) are used in communication equipment to demodulate audio or data signals from a frequency modulated (FM) or phase modulated (PM) carrier. In an FM or PM signal, the carrier signal is subtracted from the modulated signal to extract the demodulated audio or signal data. Because the carrier frequency may deviate as a result of age or environmental condition in both the transmitter and receiver, it is usually impractical to simply subtract a fixed-frequency continuous wave signal from the modulated signal. This process is complicated because the FM and PM signals in the IF signal make the carrier appear to have a constantly varying frequency as seen by the demodulator circuitry.
Because of this constantly varying carrier frequency and narrow bandwidth, each demodulator is manually aligned so that the nominal carrier frequency of the IF input signal is in the center of the demodulator's lock-in range. In the past, this alignment has been performed by the demodulator circuit manufacturer. It is desirable to have a demodulator that can perform the alignment process automatically. Also, because the manual alignment is an expensive procedure, realignment of an older receiver in the field was cost prohibitive and impractical.
A PLL demodulator is different from a PLL local oscillator (LO) used for frequency synthesis in a radio receiver. In a local oscillator circuit, the PLL is used to synthesize a reference frequency that is applied to a mixer to generate an intermediate frequency (IF) signal. In contrast, a demodulator serves to extract the data or audio signal from the IF signal. Hence, a demodulator desirably has a narrow bandwidth and very small lock-in range in which phase lock can occur. Because of the narrow bandwidth, the demodulator will not operate unless it is precisely aligned.
It is desirable to form PLL demodulators both as separate integrated circuits (ICs) and as complex circuits integrated with many receiver components integrated on a single IC chip. Complementary metal oxide semiconductor (CMOS) technology is well known as a cost efficient technology for mass production of electronic circuits. CMOS devices use minimal power when they are in a static (i.e., non-switching) state. Commercially available CMOS processing circuits including microprocessors and digital signal processors are often used to control receiver functions. Because these processors are already available in the receiver, it is both cost efficient and power efficient to implement additional demodulator functionality using circuitry already existing in the receiver controller processor.